Synthesis, Characterization, and Reactivity of Neutral Octahedral Alkyl-Cobalt(III) Complexes Bearing a Dianionic Pentadentate Ligand

A variety of neutral alkyl-cobalt­(III) complexes bearing a dianionic tetrapodal pentadentate ligand B2Pz4Py are reported. Compounds 2-R (RCH3, CH2SiMe3, CH2SiMe2Ph, iBu, CH2(c-C5H9), and (CH2)4CHCH2) are synthesized in 58–90% yield. These diamagnetic, octahedral complexes are thermally stable up to 110°C and are also remarkably stable to ambient atmosphere. They were fully characterized by spectroscopic technique and, in three cases, by X-ray crystallography. Evidence for reversible homolytic cleavage of the Co–C bonds was found in their reactions with the hydrogen atom donor 1,4-cyclohexadiene and the radical-trap TEMPO, as well as the observed cyclization of the 5-hexenyl group to the methylcyclopentyl derivative over the course of several hours. Despite these observations, it can be concluded that the diborate B2Pz4Py ligand provides a very stable platform for these Co­(III) alkyls. Reduction by one electron to a Co­(II) alkyl can accelerate bond homolysis, but in this instance, using cobaltocene as the reducing agent leads to ejection of an alkide anion through bond heterolysis, an unusual reaction for Co­(III) alkyls. Finally, protonation of compound 2-Me with the strong acid HNTf2 leads to divergent reactivity in which the major protonation site is the pyridyl nitrogen of the ligand as opposed to protonation of the methyl group. The product of protonation at nitrogen is the dimeric species 4 which was prepared via separate synthesis and characterized by X-ray crystallography.